Let call member events expire (#3831)

* Expire m.call.member events after 1 hour

* Allow clients to choose their own timeouts

proposals/3401-group-voip.md

@@ -99,9 +99,11 @@ We mandate at most one call per room at any given point to avoid UX nightmares -
 
 ### Call participation
 
-Users who want to participate in the call declare this by publishing a `m.call.member` state event using their matrix ID as the state key (thus ensuring other users cannot edit it).  The event contains an array of `m.calls` object describing which calls the user is participating in within that room.  This array must contain one item (for now).
+Users who want to participate in the call declare this by publishing a `m.call.member` state event using their matrix ID as the state key (thus ensuring other users cannot edit it).  The event contains a timestamp named `m.expires_ts` describing when this data should be considered stale, and an array `m.calls` of objects describing which calls the user is participating in within that room.  This array must contain one item (for now).
 
-`m.call.member` state events must be ignored if their user's `m.room.member` event's membership field is not `join`.
+When sending an `m.call.member` event, clients should choose a reasonable value for `m.expires_ts` in case they go offline unexpectedly. If the user stays connected for longer than this time, the client must actively update the state event with a new expiration timestamp.
+
+`m.call.member` state events must be ignored if the `m.expires_ts` field indicates they have expired, or if their user's `m.room.member` event's membership field is not `join`.
 
 The fields within the item in the `m.calls` contents are:
 
@@ -183,7 +185,8 @@ For instance:
                     }
                 ]
             }
-        ]
+        ],
+        "m.expires_ts":  1654616071686
     }
 }
 ```